1. Field of the Invention
The present invention relates generally to systems and methods for sculpting materials, and more particularly to a laser ablation system and method for sculpting a lens in a cornea.
Lasers have been used for several years to sculpt materials into very precise shapes. Excimer lasers are now widely used to ablate tissue in a variety of surgical procedures, particularly for corneal ablation during refractive surgery. The exposure of the tissue is typically controlled to produce a desired change in corneal shape. The change in corneal shape may be intended to correct a refractive error of the eye so as to eliminate the need for corrective eyeglasses, or may be intended to remove a pathology from the eye.
Known laser eye procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of stromal tissue from the cornea of the eye to alter its refractive power. The laser removes a selected portion of the corneal tissue, often to correct refractive errors of the eye. Laser ablation results in photodecomposition of the corneal tissue, but generally does not cause significant thermal damage to adjacent and underlying tissues of the eye. The irradiated molecules are broken into smaller volatile fragments photochemically, directly breaking the intermolecular bonds.
Selective photoablation of corneal tissues benefits from precise control over a laser beam. Control over the distribution of the ablative laser energy across the cornea may be provided by a variety of systems and methods, including the use of ablatable masks, moveable apertures, scanning systems that move laser beams of varying cross-section across the cornea, and the like. These laser control systems generally vary the profile of the laser beam, and thus the ablation area on which the laser impinges on the eye. As the ablation depth generally varies with the amount of laser energy, the distribution of laser energy across the laser beam is often kept as uniform as possible. The goal of this uniform energy distribution is to remove the corneal tissues uniformly throughout the laser cross-section. As excimer lasers produce laser beams as a series of laser pulses, the total ablation is often calculated as a series of ablations of uniform depth.
For laser refractive surgery to have an optimal result, the sculpting process should accurately remove corneal tissues so as to change the refractive characteristics of the eye in the desired manner. The tissues targeted for removal will generally be lens-shaped, and this lens-shaped ablation should often be surrounded by a smoothly tapering transition zone. Such a total ablation can only be approximated by the series of pulse ablations produced with most pulsed excimer lasers. This can result in ablations having undesirably abrupt changes in depth and/or staggered edges.
Several techniques have been proposed to smooth ablations. One proposal is to smooth the sharp edge of an ablation formed from an imaged aperture by defocusing the laser beam. An alternate proposal is to move the laser beam across the corneal surface between pulses so that the sequential pulses only partially overlap. Although refractive laser surgery using such approaches might be effective, the final ablations can often be less smooth than is desired. Known methods for defocusing of the laser beam may also reduce the accuracy of the overall refractive correction. Although partially overlapping sequential laser pulses can prevent the ablation edges of separate pulses from lining up, the size of each pulse edge is unaffected. Additionally, work in connection with the present invention has found that the precise shape of the actual ablation produced by a uniform laser pulse generally differs somewhat from the uniform ablation depth that has been theoretically predicted. Hence, the total ablation region can differ significantly from even the approximate lens shape that is intended.
In light of the above, it would be desirable to provide improved laser systems and methods for sculpting with lasers. It would be particularly desirable to provide new techniques for smoothing the ablations produced by lasers, especially the corneal ablations of laser refractive surgery. It would further be desirable if these improved techniques minimized unintended variations in the ablation depth, and did not significantly add to the cost or complexity of the laser systems.
2. Description of the Background Art
The following references are herein incorporated by reference in their entirety: U.S. Pat. No. 5,646,791 for “METHOD AND APPARATUS FOR TEMPORAL AND SPATIAL BEAM INTEGRATION;” U.S. Pat. No. 5,683,379 for “APPARATUS FOR MODIFYING THE SURFACE OF THE EYE THROUGH LARGE BEAM LASER POLISHING AND METHOD OF CONTROLLING THE APPARATUS;” U.S. Pat. No. 5,610,733 for “BEAM-HOMOGENIZER;” U.S. Pat. No. 4,547,037 for “HOLOGRAPHIC METHOD FOR PRODUCING DESIRED WAVEFRONT TRANSFORMATIONS;” U.S. Pat. No. 5,685,998 for “METHOD OF MINIMIZING DIFFRACTION GROOVE FORMATION ON LASER ETCHED SURFACES;” and U.S. patent application Ser. No. 08/968,380, for “METHOD AND SYSTEM FOR LASER TREATMENT OF REFRACTIVE ERRORS USING OFFSET IMAGING,” as filed Nov. 12, 1998.
The publication “DIFFRACTIVE SMOOTHING OF EXCIMER LASER ABLATION USING A DEFOCUSED BEAM” by McDonnel et al., published in Refractive and Corneal Surgery, Volume 10 (January/February 1994) describes a technique for smoothing ablations and is herein incorporated by reference in its entirety. An article entitled “AXIAL AND TRANSVERSE DISPLACEMENT TOLERANCES DURING EXCIMER LASER SURGERY FOR MYOPIA” by Shimmick et al., SPIE Ophthalmic Technologies, Volume 1423, page 140 (1991) may be relevant, and is also incorporated herein by reference.